First, definitely clean up and fix that ground. I had a poor ground on the engine block which almost caused a fire as it heated up and melted the insulation on another wire passing beside it, besides having spent considerable time debugging a non-charging alternator (it can be confusing when the alternator +ve output is correct, but its -ve terminal is actually a couple of volts above "chassis ground", to which all measurements were referenced, hence very little current flow.)

A separate starting battery is best; even with a good battery I had to move my Link2000 battery monitor power source to the house bank (3 group 27) because it would often register a "low battery" alarm when starting my 4JH2-TE.

Another approach bif you really can't fit a starting battery is what a friend did on his boat: to prevent his VHF from resetting when when cranking his engine, he added a small gel-cell alarm-type battery with a blocking diode from "ship's battery" to the backup battery, then to the radio. It does incur a half-volt (0.7v for purists, less if he used a Schottkey diode, I didn't ask) drop but the radio now stays on while cranking. Plus it give him a bit of "emergency power" for the VHF to call for a tow if he flattens his main bank while attempting to start, at least theoretically.

The simplest thing would probably be for you to split your paralleled 3-battery bank into two; one for starting and the remaining two for a house bank. You'd only need a minimal amount of cabling to separate the panel and engine feeds and to add a battery combiner relay or similar (ACR) for charging. You'd have a smaller house bank but avoid the need to find or create space for another starting battery.

I'd also double-check the power connections to the chartplotter, maybe it's only getting marginal voltage to begin with.

As another poster wrote, load-testing the batteries would be a good idea - the 4JH is a big engine, but that's a big battery bank.